This invention relates to a method and apparatus for determining the specific gravity of liquids and more particularly relates to a method and apparatus for accurately determining the specific gravity of liquid and solution process streams.
The specific gravity of liquids can be measured by using any one of a large number of methods and apparatus. One of the simplest of these is the use of a variable immersion hydrometer placed directly in the liquid whose specific gravity is to be measured. The degree of immersion of the hydrometer is an indication of the specific gravity of the liquid. The accuracy of the measurement and of the specific gravity are a function of such liquid properties as surface tension, turbulence, composition, purity and temperature.
The variable immersion hydrometer may be positioned in a rotameter housing and may be equipped with an electronic circuit based on measurement of impedance or capacitance. Automatic temperature compensation may also be provided.
The prior art discloses many methods for measuring the specific gravity of liquids based on the variable immersion hydrometer equipped with electronic transmission of a signal proportional to the specific gravity of the liquid. However, the methods and apparatus are usually complex and often have a limited accuracy. For example, in U.S. Pat. No. 3,392,589 which issued on July 16, 1968 to L. E. Kuntz et al., there is disclosed an apparatus for continuously determining the specific gravity of a product stream which comprises a constant overflow chamber, a hydrometer, an electrical capacitance variable by the hydrometer in relation to its immersion, and means for translating the capacitance into a signal representative of the liquid density. The apparatus also includes means for sensing liquid temperature and generating a further signal which, in combination with the density representative signal, forms a third signal representative of the specific gravity of the liquid at a predetermined temperature. This method is complex as it necessitates the translation of the capacitance into another signal.
In many processes it is necessary to measure the specific gravity or strength of process liquids with great accuracy. The ability of the electrical, nuclear and other devices to measure specific gravity accurately depends greatly on the signal to noise ratio of the device. Where this ratio is high, a high degree of accuracy can be attained. However, most measuring devices have a low ratio and, in addition, have high drift characteristics which make frequent calibration necessary.
Another prerequisite for the accurate measurement of liquid specific gravity is the need for preventing undesirable movements of the hydrometer. Thus, a high degree of accuracy is difficult to obtain in measurement in a continuous fashion in a flowing process stream, as this entails measurement in a moving body of liquid which is usually subject to fluctuations in the rate of flow of the liquid. Measurement in a batchwise fashion, for example, by withdrawing a sample and measuring the specific gravity in the sample, is not only time consuming but makes control difficult, although the desired degree of accuracy might be obtained.
Still another prerequisite is the need for linearity of the measuring system. Many measuring systems, such as for example Wheatstone bridges, do not usually provide the required degree of linearity and require compensating means.